Organic reactions can be described in terms of its kind and how they occur. In terms of its kind, there are four common kinds of organic reactions, namely:
(McMurry, 8th ed).
Kinds of Organic Reactions
Addition reactions occur when two reactants combined to form one product. Halogenation of ethene is an example of this kind of reaction.
Substitution reactions occur when one reactant exchange one atom with the other to form two products. An example of this kind is the reaction of tert-butyl alcohol with hydrochloric acid, HCl, in the presence of zinc chloride, ZnCl2.
Elimination reactions are, in a way, the opposite of addition. Here atoms from a reactant are removed to form two products. Dehydrohalogenation of an alkyl halide to form an unsaturated hydrocarbon is an example of this reaction.
Rearrangement reactions requires movements of the bonds of a single reactant to form a new product. Claisen rearrangement of ethers at high temperature is an example of this reaction.
Organic mechanism provides a more detailed description on how reactions happen. It shows when bonds are broken and formed. When studying organic mechanism, it is easier to group reactions into two: radical and polar reactions. This classification is based on how bonds are broken and formed.
Radical reactions involved formation of free radicals, resulting from the homolytic (symmetrical) cleaving of covalent bonds. A radical is a very reactive chemical species with an odd number of electrons.
Polar reactions involved a heterolytic (unsymmetrical) cleavage of the covalent bond, resulting to the formation of charged chemical species. This occurs when one of the atoms involved in the bond is more electronegative than the other. The more electronegative atom would polarize the bond, resulting to cleavage and acquisition of a negative charge.
To better understand reaction mechanism, an energy profile diagram is used (see below). At a glance, one can determine how many steps are involved in the reaction, its rate determining step, and its enthalpy.
The following discussion will illustrate two examples of reaction mechanism that involved homolytic and heterolytic cleavage of bonds, respectively.
The mechanism of a radical reaction involves three steps: initiation, propagation, and termination. Halogenation of ethane in the presence of light will be used as example to illustrate the steps of radical reactions.
In this step, highly energized light particles (e.g. ultraviolet) are introduced to initiate symmetrical cleaving of the bond. This results to the formation of the radical.
Cl-Cl 2 Cl
As a highly reactive chemical species, the radical abstracts atoms to form another radical species. As more radicals are generated, the reaction cascades like a chain.
When a radical encounters another radical, the reaction stops.
Aside from the manner on how bonds are broken, polar reaction can best be described as a reaction between nucleophile and electrophile.
A nucleophile (nucleus loving) is a chemical species with a highly dense electron site. These are Lewis bases or negatively-charged ions. Examples of these nucleophiles are NH3 and CH3O1-.
An electrophile is a chemical species with a low dense electron site. These are Lewis acids or positively-charged ions. Examples are AlCl3 and Br1+. A free radical can also be classified as an electrophile because of its odd electron.
An example of a polar reaction is the reaction of 2-bromo-2-methylpropane and water.
In this particular reaction mechanism, arrows will be used to show the movement of electrons. It has to be noted that an electron rich site attacks the electron deficient site, as shown by the arrows.
A thorough knowledge of the mechanism of a reaction gives a chemist the advantage of manipulating the conditions of the reaction so as to serve a particular purpose. Take the reaction of 2-bromo-2-methylpropane and water. This reaction is an example of nucleophilic substitution unimolecular (S N1).
The energy profile diagram of this reaction would be like this:
Several characteristics of the reaction can be derived from its energy profile diagram. These include: (1) The reaction is a two-step mechanism; (2) The first step is the rate-determining step as it requires bigger activation energy than the second step; and (3) The first step involves the formation of the intermediate carbocation.
Since the reaction is a two-step mechanism and its first step is the rate-determining step, the rate law of this particular reaction can be expressed as:
Rate = k[RX]
Despite the presence of the nucleophile at the start of the reaction, it does not interact yet with the alkyl halide until the second step. Since the rate law involves only the alkyl halide, the rate is said to be unimolecular.
As the alkyl halide formed a carbocation, a negatively-charged species would interact with it, which is referred as a nucleophile. Its mechanism also indicates that the nucleophile takes the original place of the leaving group halide. Hence, the reaction is called nucleophilic substitution.
Knowledge of the rate law of the reaction allows chemists to determine means that will increase the rate of the reaction. Several ways can be done to increase the rate of this particular reaction.
The first one is the increase the concentration of the alkyl halide. As indicated by the rate law, a more concentrated alkyl halide results to an increase in the reaction rate.
Another technique to increase the rate of the reaction is to generate a condition that leads to the stabilization of the intermediate, carbocation. One way is to use polar protic solvents, like water and acetone. These solvents are capable of forming hydrogen bond and their negative centers stabilized the carbocation.
Other option is increasing the reactivity is by changing the halide found in the reactant. The halide is the leaving group in this particular reaction. The term leaving group refers to an atom or a group of atoms that leaves the substrate to form the intermediate/transition state. Oftentimes, this term is used during nucleophilic substitution and elimination, although other mechanisms used this as well.
In unimolecular nucleophilic substitution, the I1- is the best leaving group, followed by Cl1-, Br1- and F1-.
Leaving group which are weak base are best as they don’t go back to react with the carbocation. Therefore, to improve the yield of this reaction, ter-butyl chloride can be replaced by tert--butyl iodide.
Electrophilic Aromatic Substitution (SE-Ar)
Aromatic compounds, such as benzene and toluene, mostly react through electrophilic aromatic substitution. Since these compounds are stabilized by resonance effect, they don’t easily react. A catalyst is needed to prepare the electrophile that readily reacts with the aromatic ring.
Friedel-Crafts alkylation is an example of an electrophilic aromatic substitution. An example is the reaction of benzene and ethyl chloride in the presence of aluminum chloride to form ethyl benzene and hydrochloric acid.
The equation requires four steps in its mechanism: (a) generation of the electrophile, (b) the formation of the carbocation intermediate, (c) restoration of the aromatic character of the ring, and (d) regeneration of the catalyst.
The first step involves the polarization of the alkyl halide by the aluminum chloride, a Lewis base. This leads to the generation of the electrophile.
CH3CH2 – Cl + Cl – AlCl2 CH3CH21+ + AlCl41-
The electrophile, CH3CH21+, is then attacked by the nucleophilic benzene ring to form the carbocation intermediate.
The aromatic character of the ring will be restored when a C – H is cleaved.
Lastly, the catalyst is regenerated when the H1+, liberated after the aromatic ring is restored, reacts with AlCl41-.
H1+ + AlCl41- H-Cl + AlCl3
The energy profile diagram of electrophilic aromatic substitution only indicates the two crucial steps in the mechanism, i.e. the attack of the ring to the electrophile forming the carbocation intermediate, and the formation of the product.
Since the formation of the carbocation requires more activation energy than the second step, this will be the rate-determining step. Its rate law can be expressed as:
Rate = k[benzene][CH3CH2Cl]
This indicates that the rate of the reaction is affected by the concentrations of the aromatic substrate and the electrophile. Increasing either concentrations or both will lead to faster reaction.
The structure of aromatic ring also has a consequence. If a substituent of the ring is an electron-donating group, such as ‒CH3, ‒OH, ‒OCH 3, and ‒NH2, can increase the rate of the reaction too. These groups stabilized the carbocation intermediate, making its formation more feasible.